Device for determining the end of the processing time for hardening masses

ABSTRACT

A device is provided which determines the end of the processing time for hardening masses, in particular dental molding masses. The device comprises a display unit and a sensor unit which record the change in at least one of the rheological properties of the mass. A method that can be used with such a device is described.

FIELD OF THE INVENTION

[0001] The invention relates to a device and a method for determiningthe end of the processing time of hardenable compounds, in particular ofdental impression compounds.

BACKGROUND AND SUMMARY OF THE INVENTION

[0002] To process and use impression compounds in dentistry, thedirections for use normally specify suitable times, how the impressioncompound is to be handled, and when it is to be removed from thepatient's mouth.

[0003] However, the specified times for processing and the settingbehavior of the compounds are subject to various influences in dentalpractice, such as the actual mouth temperature and room temperature, themixing energy applied, or the time which was spent on the mixing.

[0004] It is conceivable to incorporate indicators into thepolymerizable compounds, which indicators are released during thepolymerization and indicate the progress of the reaction, for example bya change in the color intensity. Such an attempt is described inWO-96/00560.

[0005] A disadvantage of this is that the compounds described contain afurther component which may have a negative effect on the desiredproperties. In clearly definable signal of the setting process and itadditionally requires continuous visual monitoring.

[0006] DE 29 906 343 U1 attempts to solve the problem by makingavailable a unit for dispensing multi-component compounds, which unit isequipped with a timer clock for specifying a time relevant for theprocessing.

[0007] The unit described has the disadvantage, however, that the mixedcompound is again subject to said external influences and, therefore, noclear indication can be given of when the setting process begins.

[0008] Moreover, industrial measuring mixers for checking theconsistency of cement/water mixtures are known which are used forpreparing concrete (DE 199 03 753 C1).

[0009] An ancillary device for torque measurement for stirring units, inparticular for small-scale fermenters, is described in DE 28 50 486 A1.

[0010] DE 39 19 534 A1 discloses a method and a device for preparingbone cement. A propeller is lowered into a vessel in which the bonecement is to be prepared. The preparation of the bone cement isprocess-controlled.

[0011] None of the devices known from the prior art is suitable fordetermining the change in rheological properties of comparativelyquickly hardening compounds, in particular dental impression compounds,which have to be prepared in portions and made available at shortintervals one after another.

[0012] It is therefore an object of the present invention to propose adevice and a method which solve the problems discussed above and informthe user of the progress of the curing of hardening compounds, inparticular of dental impression compounds.

[0013] This object is achieved by a device and a method as are describedbelow.

[0014] The term “setting profile” generally describes the changes inrheological properties occurring during the hardening or setting ofhardenable compounds. The determination of the end of the processingtime is closely linked to this.

[0015] The expression “processing time” is to be understood as the timeduring which the mixed compound can still be used in the intendedmanner. In the case of dental impression compounds, this is the time upto which the impression compound can be relatively easily lifted fromthe article of which an impression is to be made, without appreciablyimpairing the accuracy of the impression. This is usually the point intime at which the compound changes from the pasty, plastic state to atough and resilient, rubber-like state.

[0016] The expression “hardenable compounds” includes all compoundswhich, as a result of a polymerization reaction, for example afree-radical, cationic or anionic addition reaction and/or condensationreaction and/or cement reaction, can change from a viscous, flowable,optionally plastically deformable state to a permanently shaped, rigidstate.

[0017] Compounds whose setting process can preferably be recorded withthe device according to the invention usually have the followingproperties before the start of the setting process. They are pasty,highly viscous substances which rigidify over a period in the range of0.1 to 60 minutes, preferably 1 to 8 minutes. Before the start of thesetting process, such compounds have, for example, a viscosity of class0 to 3 as determined by the DIN 4823 consistency test, measured with adiameter of less than 80 mm. Materials which may be mentioned aspossible examples are silicones, polyethers, epoxy resins andpolyurethanes.

[0018] The Shore hardness A of the compounds measured according to DIN53505 fifteen minutes after the end of the processing time usually liesin the range of 20 to 110, preferably in the range of 30 to 80.

[0019] The invention is suitable in particular for recording the settingbehavior of sealing compounds and dental compounds, preferably dentalimpression compounds, for example those based on polyethers, A and Csilicones, alginates and/or polyether silicones.

[0020] “Rheological property” is to be understood as meaning allproperties whose change can be recorded via a physical and/or chemicalmeasurement method. These include, in particular, properties such asviscosity, compressive strength, pH, conductivity, dielectric constant,impedance, capacitance, hardness, density and/or temperature.

[0021] In contrast to static mixers, the term “dynamic mixer” includesmixers which have at least one rotatably mounted part which is drivenvia a mixer shaft. Such mixers are described, for example, in DE 90 17323 U1 or WOA-98/43727. These mixers are what are referred to ascontinuous-flow mixers. Mixed material or paste is dispensed from themixer during the mixing, whereas, during the measurement, the at leastpartially mixed material remains in the mixer and is not conveyed anyfurther or dispensed.

[0022] A dynamic mixer generally comprises the following componentparts: a housing, inlet openings, an outlet opening, and a rotatablymounted mixing element with mixing vanes. It is driven either centrallyvia a rotor which has mixing vanes located on it and rotates in thehousing, or externally via a part of the housing serving as rotor and aninner body serving as stator, as is described in DE 19 947 331 A1.

[0023] Since mixers of this kind are usually disposable articles whosevalue is generally considered slight when compared to the substanceswhich are to be mixed, these mixers have a relatively small mixingvolume in order to keep the amount of mixed compound to be discarded assmall as possible. Such disposable articles are usually made of plastic.

[0024] A sensor unit within the meaning of the invention is a unit whichis able to record any change in state of the impression compound via acoupling to the mixer shaft. This includes capacitors, ultrasoundsensors, pH electrodes, torque transducers, quartz oscillators,thermoelements, current meters, voltage meters, resistance meters, wirestrain gages and force transducers.

[0025] The term “display unit” includes all units which are able toinform the person using the device of a change in the state of theimpression compound, preferably in optical and/or acoustic form. Thisincludes displays, in particular LED displays, and loudspeakers.

[0026] The terms “comprise” and “contain” introduce a nonexhaustive listof features. The fact that the word “a” is used before the mention of afeature in the claims does not exclude the possibility that saidfeatures can be present more than once, in the sense of “at least one”.

[0027] The invention has the following advantages:

[0028] The device according to the invention and the method according tothe invention permit immediate monitoring of the process of hardening ofthe mixed compound, substantially independently of external influences.

[0029] In contrast to measurement mixers, which are known from the priorart, the change in rheological properties is not determined on the basisof a material sample which is subsequently further used, but instead onthe basis of a material sample which remains in the mixer and isdiscarded together with the latter.

[0030] The measurement can be carried out directly in the devicenormally used for mixing and dispensing the hardening compounds.Arranging additional auxiliary means, such as a bypass, chambers,volumetric flask, measurement drives or probes, on known measurementdevices is not necessary.

[0031] If the measurement takes place from the time at which no furthermixed compound is dispensed through the mixer, the invention makes itpossible to record the hardening process of the last-mixed compound inthe mixer itself, which is usually discarded after the compound hashardened. The information as to when the last-mixed compound begins toharden is, in particular, of not inconsiderable importance to the user,for example the dentist, in preparing an impression of the hard dentaltissue using impression compounds.

[0032] This represents a decisive advance over the method known from DE299 06 343 U1, in which the time up to which the compound can beprocessed is predefined and thus not linked to the actual settingprocess.

[0033] The present invention thus means that dynamic mixers which areused to mix hardening compounds can be used, after the mixing process,to monitor the setting profile of the hardening compounds. The device isnot limited to a certain type of mixer. Different mixers with differentgeometries can be used depending on the compounds which are to be mixed.A mixer can be changed very easily, and this can be done in less thanone minute.

[0034] The compounds are usually prepared by mixing a base paste and acatalyzer paste. The speed of the setting process differs depending onthe mixing ratio and on the substances. However, the device according tothe invention is also suitable for determining the setting process of amixture of three or more pastes or substances. In addition topaste/paste mixtures, the mixtures can also be paste/fluid mixtures orpowder/fluid mixtures.

[0035] The pastes are mixed in a dynamic mixer, for example according toDE 90 17 323 U or WO-98/43727. This mixer is usually placed on acorresponding cartridge and driven by an electrically actuated mixingunit into which the cartridge can be fitted. Suitable mixing units aredescribed in EP 0 492 413 A.

[0036] If the compound obtained by mixing is a dental impressioncompound, this is preferably used to fill a dental impression tray whichis then fitted in a patient's mouth.

[0037] Since in this case the setting profile according to the inventionis measured outside the mouth and the profile is temperature-dependent,the setting of the compound in the unit takes place somewhat more slowlythan in the mouth. This ensures that in every case the compound hascompletely set before it is removed from the mouth.

[0038] The direct correlation between the setting profile in the mouthand in the mixing unit can be determined empirically as a function ofthe compound used.

[0039] The measurement of the change in a rheological property of thehardening compound and the comparison with a predeterminable thresholdvalue are carried out using a measurement program which can beintegrated into the control of the mixer shaft or is operatedindependently of the latter.

[0040] The measurement is preferably carried out directly via the mixershaft of an electrically operated mixing unit, said mixer shaft drivinga dynamic mixer. After an impression tray has been filled, the mixershaft continues to be driven with the forward feed switched off, so thatthe torque applying on the shaft increases as the compactness of thecompound increases. In this embodiment, the end of the processing timeof the compound can be indicated by a visual or acoustic signal as soonas a determined threshold value is reached which can be individually setdepending on the compound or is automatically set as soon as thecartridge containing the compound is placed in the mixing unit.

[0041] The mixer shaft is coupled, for example, to a torque transduceror a rotatable shaft which make it possible to determine the torqueapplied to the mixer vanes of the mixer or to the mixer shaft, whichtorque is proportional to the viscosity of the hardening compound. It isalso possible to record the change in torque via the current consumptionof the drive.

[0042] The change is preferably measured with the mixer shaft moving ata reduced speed of rotation compared to during mixing, in order not tounnecessarily introduce rotation energy into the hardening compound,which can lead to an undesirable increase in temperature. Thisadditionally permits a more precise determination of the change. Speedsof rotation which can be used for the mixer shaft during the measurementprocess lie in the range of 10⁰ to 10³ and preferably in the range of10¹ to 10² rpm.

[0043] In an alternative embodiment, the measurement process takes placevia the mixer shaft in a pulsed mode or cyclically. For example, themeasurement takes place only every 1, 5 or 10 seconds.

[0044] The measurement of the change in a rheological property of thehardening compound can also take place if the mixer shaft does not fullyrotate. A turning of the mixer shaft in an angle range of 1 through180°, preferably 10 through 90°, has proven advantageous.

[0045] However, another example of a suitable measuring unit is a quartzoscillator which generates a purely torsional oscillation, the latterbeing damped by the viscous properties of the hardening compound. Thesensor head can be arranged in the form of a probe at or in the area ofthe tip of the mixer shaft and can be introduced into the compound via aseal in the area of the coupling of dynamic mixer/mixer shaft.

[0046] In contrast to measurement mixers known from the prior art, thepresent device not only allows a hardenable compound to be mixed anddelivered, but also makes it possible, after the mixing and deliveryprocess, to determine the setting profile of that part of the mixedcompound remaining in the exchangeable, disposable mixer.

[0047] A preferred illustrative embodiment is explained below withreference to a figure.

BRIEF DESCRIPTION OF THE DRAWING FIGURE

[0048]FIG. 1 shows a diagrammatic representation of a possiblearrangement of the individual component parts of the measurement device.

DETAILED DESCRIPTION OF THE DRAWING FIGURE

[0049] By pressing the delivery switch (1), the piston motor isactivated at a certain speed of rotation (4) via thereciprocating-piston control (2), and the piston motor moves thedelivery spindles (8, 9) and the associated pistons (10, 11) forward inthe axial direction via the drive shaft (6) and a drive element (7).

[0050] The pistons (10, 11) are guided in cartridges (14, 15) whichcontain the individual components of the hardening compound. By means ofthe axial movement of the pistons, the two components are pressedthrough the outlet openings (21, 22) of the cartridges into a dynamicmixer (16).

[0051] The mixer-shaft control (3) is activated simultaneously with theactivation of the reciprocating-piston control (2) in order to give themixer-shaft motor the speed of rotation needed for the mixing process,this speed of rotation being transmitted via the mixer shaft (19) to themixer screw (17) of the dynamic mixer (16). The mixer-shaft control canbe activated via the reciprocating-piston control or independently ofthe latter. The rotating mixing vanes (18) ensure intensive andhomogeneous mixing of the two flowing, pasty components (12, 13), sothat direct application, for example into an impression tray (not shownhere), is possible.

[0052] After the impression tray has been completely filled, thedelivery switch (1) is released, as a result of which the piston strokecontrol (2) stops the delivery process. At this moment, the mixer-shaftcontrol (3) switches to measurement mode. In other words, after themixer-shaft motor (5) has been stationary for 10 seconds, for example,the mixer shaft (19) is rotated through 180°, for example, at a slowerand constantly defined speed of rotation for torsion measurement, andthe current I required for this is measured.

[0053] As soon as the compound located in the mixer (16) begins toharden, the current intensity I needed to keep the mixer shaft at aconstant speed of rotation increases proportionally. At a certain stateof the hardening compound, a critical threshold value for the currentintensity I is exceeded, which threshold value is stored or can be inputinto the device for the respective compound, with the result that themixer-shaft control (3) activates the display unit (20), which thereuponoutputs an acoustic or visual signal.

1. A device for monitoring the setting profile of hardening compounds,comprising a mixer-shaft motor with a control, a mixer shaft, a seat fora cartridge, which can be connected to a dynamic disposable mixer whichis used to mix the hardening compound, a display unit and a sensor unit,which can record the change in at least one of the rheologicalproperties of the compound, the sensor unit being coupled to themixer-shaft motor, and the disposable mixer cooperating with the sensorunit.
 2. The device as claimed in claim 1, in which the rheologicalproperties are chosen from viscosity, compressive strength, pH,conductivity, dielectric constant, impedance, capacitance, hardness,density and/or temperature.
 3. The device as claimed in one of thepreceding claims, in which the sensor unit is chosen from: capacitors,ultrasound sensors, pH electrodes, torque transducers, quartzoscillators, thermoelements, current meters, voltage meters, resistancemeters, wire strain gages and/or force transducers.
 4. The device asclaimed in one of the preceding claims, in which the display unit canoutput a visual and/or acoustic signal.
 5. The device as claimed in oneof the preceding claims, in which the sensor unit can record the changein the torque applied to the mixer shaft.
 6. A method comprising thefollowing steps: a) mixing a hardening compound in a dynamic disposablemixer which is driven by a mixer-shaft motor, b) dispensing almost allof the compound from the dynamic disposable mixer, c) recording thechange in at least one of the rheological properties of the mixedcompound which remains in the dynamic disposable mixer via a sensorunit, d) outputting a signal as soon as the value of the change fromstep c) has reached a predeterminable threshold value, the sensor unitcooperating with the mixer-shaft motor.
 7. The method as claimed inclaim 7, in which at least step c) is performed with a device as claimedin one of claims 1 through
 6. 8. The method as claimed in one of claims7 and 8, in which, in step c), a change in the torque applied to themixer shaft can be recorded.
 9. The method as claimed in one of claims 7through 9, in which the measurement in step c) is pulsed.
 10. The methodas claimed in one of claims 7 through 10, in which the measurement instep c) takes place with a speed of rotation which is smaller than thespeed of rotation which is used in step a) for mixing.
 11. The method asclaimed in one of claims 7 through 11, in which the mixer shaft is notfully rotated during the measurement in step c).
 12. The use of thedevice as claimed in one of claims 1 through 5 or of the method asclaimed in one of claims 6 through 11 for determining the end of theprocessing time of hardening compounds.
 13. The use as claimed in claim13, in which the hardening compound is a dental impression compound. 14.The use of a dynamic disposable mixer for mixing, dispensing andsubsequently monitoring the setting profile of hardening compounds.